Computer-to-plate (CTP) is an imaging technology used in printing processes, in which image data is output directly from a computer to a printing plate by exposing the printing plate to light energy according to the image data. A printing plate is positioned on an imaging cylinder, e.g., the cylinder (also called a drum) of an external drum imaging device. As the cylinder rotates around its axis, an image head moves in the axial direction and focuses one or more laser beams modulated with image data onto an ablatable layer of the plate to ablate a pattern.
In general, printing plates may include any type of plate that is ablatable by light energy. Examples of printing plates include flexographic plates (sometimes referred to as “flexo plates”) and metal back plates (sometimes referred to as “letterpress plates”) that include a photopolymer material.
There are generally three different types of cylinder devices currently available for CTP imagers, dependent upon the types of printing plates being used. These devices include vacuum-based cylinder devices, magnetic cylinder devices and magnetic vacuum-based cylinder devices.
Conventional vacuum-based cylinder devices may be used to mount flexographic plates. They are not typically configured to mount metal back plates (such as letterpress plates). These devices include a cylinder coupled to a vacuum system. Flexographic plates are mounted to the cylinder by applying suction via the vacuum system.
Conventional magnetic cylinder devices may be used to mount metal back plates (such as letterpress plates). They are not typically configured to mount flexographic plates. The magnetic cylinder device typically includes a metal cylinder (such as aluminum) having a plurality of permanent magnets formed in the surface of the cylinder. Metal back plates are mounted to the cylinder by magnetic coupling via the permanent magnets.
Conventional magnetic vacuum-based cylinder devices may be used to mount metal back plates (such as letterpress plates) and flexographic plates. In addition to a metal cylinder having permanent magnets in the surface of the cylinder, the device includes a vacuum system coupled to the cylinder. For flexographic plates, suction is applied via the vacuum system. For metal back plates, the permanent magnets are used to magnetically couple the plates to the cylinder. When using flexographic plates on the cylinder, however, the cylinder surface (which include permanent magnets) may cause problems in an image of the flexographic plate. For example, the laser beam of a CTP imager may be reflected from the cylinder surface and may affect the image of the flexographic plate.
In general, vacuum-based cylinder devices are less expensive to manufacture than magnetic cylinder devices and magnetic vacuum-based cylinder devices. Magnetic vacuum-based cylinder devices are typically the most expensive to manufacture. The type of cylinder included with a CTP imager typically depends on the type of printing plates that will be used by a customer. For example, if only flexographic plates are to be used, the imager may be equipped with a vacuum-based cylinder device. If a user only uses metal back plates, the imager may be equipped with a magnetic cylinder device. If a user uses both metal and flexographic plates, the imager may be equipped with a magnetic vacuum-based cylinder device.
Another option for using both flexo and metal back plates on the same imager, aside from using magnetic vacuum cylinders, is to change the drums in the imager so as to use a vacuum-based cylinder device for flexographic plates and a magnetic cylinder device for metal back plates. However, this requires two drums, which is more costly than a single drum system, and may impact productivity because of the time needed to change the drums. Accordingly, there is a need for more cost effective apparatus and methods for permitting both flexo and metal back printing plates to be used on the same imager.